The present invention relates to metal oxide semiconductor imaging arrays, and more particularly, an improved charge amplifier for use in the array.
Integrated circuit technology has revolutionized various fields including computers, control systems, telecommunications, and imaging. In the field of imaging, the charge coupled device (CCD) sensor has made possible the manufacture of relatively low cost and small hand-held video cameras. Nevertheless, the solid-state CCD integrated circuits needed for imaging are relatively difficult to manufacture, and therefore are expensive. In addition, because of the differing processes involved in the manufacture of CCD integrated circuits relative to MOS integrated circuits, the signal processing portion of the imaging sensor has typically been located on a separate integrated chip. Thus, a CCD imaging device includes at least two integrated circuits: one for the CCD sensor and one for the signal processing logic.
An alternative low cost technology to CCD integrated circuits is the metal oxide semiconductor (MOS) integrated circuit. Not only are imaging devices using MOS technology less expensive to manufacture relative the CCD imaging devices, for certain applications MOS devices are superior in performance. For example, the pixel elements in a MOS device can be made smaller and therefore provide a higher resolution than CCD image sensors. In addition, the signal processing logic necessary can be integrated alongside the imaging circuitry, thus allowing for a single integrated chip to form a complete stand alone imaging device.
Examples of MOS imaging devices are detailed in xe2x80x9cA xc2xc Inch Format 250K Pixel Amplified MOS Image Sensor Using CMOS Processxe2x80x9d by Kawashima et al., IEDM 93-575 (1993), and xe2x80x9cA Low Noise Line-Amplified MOS Imaging Devicesxe2x80x9d by Ozaki et al., IEEE Transactions on Electron Devices, Vol. 38, No. 5, May 1991. In addition, U.S. Pat. No. 5,345,266 to Denyer titled xe2x80x9cMatrix Array Image Sensor Chipxe2x80x9d describes a MOS image sensor. The devices disclosed in these publications provide a general design approach to MOS imaging devices.
The primary building block of an image formed by an MOS imaging device is a pixel. The number, size and spacing of the pixels determine the resolution of the image generated by the imaging device. The pixels of a MOS imaging device are semiconductor devices that transform incident light photons into current signals. The signal produced by each pixel is generally extremely small, in the nanoampere range. This small signal is unsuitable for further processing. Thus, a critical component of a MOS image sensor is a series of charge amplifiers that amplify the signals generated by the pixel elements. It is the charge amplifier design that is the subject of the present invention.
An improved charge amplifier for use in a MOS imaging array is disclosed. The charge amplifier comprises a reference capacitor, a sense capacitor, an operational amplifier, a first output capacitor connected to the output of the operational amplifier, a second output capacitor connected to the output of the operational amplifier, and a differential amplifier connected to the first output capacitor and said second output capacitor.
In operation, the reference capacitor is charged to a reference voltage. Similarly, the sense capacitor is charged to a reference voltage. Both the sense capacitor and the reference capacitor are formed to be identical. However, the reference capacitor is covered from ambient light and the sense capacitor is exposed to ambient light. As ambient light is incident on the sense capacitor, the voltage carried by the sense capacitor diminishes. After a predetermined exposure time, both the voltage stored across the reference capacitor and the sense capacitor is amplified by the operational amplifier. Next, the differential amplifier operates to provide an output signal that is the difference between said amplified version of said reference voltage and said amplified version of said signal voltage.